In automotive and other industrial applications, special sensors are used to determine shaft speed and angular position, for example, as well as linear motion sensing. Generally, such sensors are of the variable reluctance variety and comprise a toothed wheel spaced from a sensor comprising a magnet and a magnetoresistor or a Hall effect device. Other types of sensors require multi-bit digital encoding for position sensing and other uses.
A permanent magnet with an appropriate magnetization pattern can serve as the exciter component of a magnetoresistive sensor without the need for a separate bias magnet. However, by conventional production methods currently in use, very small magnet exciters could not be magnetized with a pattern providing the necessary resolution, and the cost of a large permanent magnet exciter would be prohibitive. If several different magnetization patterns are desired side by side, such as for multi-bit digital encoding, more complex manufacturing problems arise; either machining or magnetizing such an exciter as one unit is very costly and is seldom done.
It has been proposed in U.S. Pat. No. 4,312,684 to Chraplyvy et al entitled "Selective Magnetization of Manganese-Aluminum Alloys" and in U.S. Pat. No. 4,347,086 to Chraplyvy et al entitled "Selective Magnetization of Rare-Earth Transitional Metal Alloys", both assigned to the assignee of the present invention, to create local regions of hard magnetic material in a substrate of a special non-magnetic or soft magnetic material by exposing selected regions of the substrate to a laser beam for heating such regions to a transformation temperature at which magnetic material is formed. The magnetic regions are magnetized in a strong field to produce a permanent magnetic code having sufficient flux density to be readable with a magnetic sensor such as a magnetic tape head. The materials used are expensive and the magnetic fields produced are too weak for sensing by non-contact sensors at a distance greater than about 380 .mu.m.
In addition, the paper of Ara et al, "Formation of Magnetic Grating on Steel Plates by Electron/Laser Beam Irradiation", IEEE Trans. Magnetics, Vol. 25, No. 5 (1989), p. 3830, discloses an attempt to make a magnetic sensor by forming magnetic gratings on non-magnetic austenitic stainless steel by laser beam heating of strips on the plate to a temperature sufficient to effect transformation of the heated regions to produce small grains of the ferromagnetic phase in the austenitic phase, and similarly heating a ferromagnetic carbon steel having a ferrite/pearlite phase which was changed to martensite by beam irradiation. The gratings were magnetized and the magnetic flux from each track was detected by a sensor passed over the grating. The signal produced was far too weak to be useful in many applications.
It has also been proposed to alter the magnetic properties of very thin films of special materials for data storage by a thermomagnetic method. In the recording of a magneto-optical disc, a thin layer (about 1 .mu.m thick) of an amorphous transition metal-rare earth alloy is coated on a disc and the entire disc is magnetized in a given direction. A laser is then used to locally heat the surface (typically, a 1.6 .mu.m diameter spot) in a static applied magnetic field to reverse the direction of the disc's magnetization in the heated region. Because the magnetic regions are so small and magnetically weak, a magnetic sensor such as a magnetoresistor or a Hall effect device cannot respond to the individual bits of data. The data is read optically using the Kerr effect. This requires a beam-splitter, two detectors, two linear polarizers, a half-wave plate, and beam steering optics. The delicate and complex nature of the detection optics precludes this type of magneto-optical recording from forming the basis of a viable automotive sensor.